1. Field of Invention
The present invention relates to a semi-conductor memory, and more particular, to a multilevel phase-change memory
2. Related Art
Most electronic equipment uses different types of memories, such as a DRAM, SRAM and Flash memory or the combination of these different types' memories based on the requirements of the application, the operating speed, the memory size and the cost consideration of the equipment. The current new developments in the memory field include FeRAM, MRAM and phase-change memory.
A Phase-change memory records data by changing the material of the semi-conductor circuit to different phase states due to the resistance changes inside of the circuit. Many materials, such as Ge2Sb2Te5 and others, have the characteristics of changing to different crystallization states with different temperatures and different crystallization states have different resistance. Therefore, a phase-change memory using electrical heating can change materials, such as Ge2Sb2Te5, into different crystallization states, each having different resistance, where each different state can represent a different recording state, i.e., 0 or 1. Moreover, a phase-change memory is non-volatile, it will retain data recorded even if the power is off. Therefore, during a data writing operation in a phase-change memory, the electrical current has to be supplied to the selected memory cells, which will cause phase transition after being heated up by heating electrodes. The current phase-change memory technology uses contact to make a phase area method, such as a structure combining phase-change memory and a CMOS transistor disclosed by Samsung Electronics Co., Ltd in IEDM 2003, which connects heating electrodes to the phase-change layer and uses the contact as a phase-change area. Also in IEDM 2003, STMicroelectronics & Ovonyx Inc. recommended another structure using the via as phase-change region, which fills the phase-change layer in the via to obtain a smaller switching current.
The above-referenced technology using the via to connect a heating electrode to a phase-change region will prevent making a high capacity memory. Therefore, the increase of a phase-change memory density is one key focus in the current memory technology development.